At more than 2,600, the U.S. now leads the world in confirmed cases of swine flu.
Mexico isn't far behind with more than 1,600 confirmed cases, but health experts still believe that Mexico will ultimately lead the world in total cases once all is said and done.
The U.S. Centers for Disease Control and Prevention reports that there have been 2,618 flu cases in 44 states with three deaths.
The number of cases of swine flu may have been several times higher than reported and the potential for rapid spread of the illness justified the World Health Organization's decision to raise the global pandemic alert, a new study concludes.
While about 4,800 confirmed cases have been reported in 30 countries, the new analysis estimates there have been between 6,000 and 32,000 cases in Mexico alone.
While there have been 1,626 cases of the flu confirmed in Mexico, the researchers note that there have been more than 11,000 suspected infections.
TheCDC has not offered an estimate of how many total swine infections have hit the United States so far. However, the number of confirmed cases is no doubt just "the tip of the iceberg," said Dr. Anne Schuchat, the CDC's interim deputy director for science and public health.
About 40 percent of confirmed U.S. flu cases are probably swine flu, according to CDC data from about nine days ago.
"Our early analysis would suggest this is going to be an outbreak comparable to that of 20th century pandemics regarding the extent of its spread — it's very difficult to quantify the human health impact at this stage, however," said lead author Neil Ferguson of Imperial College, London.
Ferguson's analysis was released by the journal Science. Normally Science releases its reports on Thursdays but the journal said it was issuing this study early because it contains important public health information.
Meanwhile, on Monday, the World Health Organization's assistant director-general moved to defend the global body against accusations that it stoked unnecessary fear over a flu outbreak that appears to be relatively mild.
"I hope we have come across as trying to present a very balanced picture," Keiji Fukuda told reporters in Geneva.
"I think that one of the things we made clear is that the future is not possible to predict and there are many ways that events could turn out."
"Things could stay relatively mild, things could become more severe. Both of these are possible," he said. "I think without that information both people and countries cannot prepare as well as they can."
Ferguson's researchers said the 2009 H1N1 flu appears to be about equal in severity to the flu of 1957 and less severe than the deadly 1918 version.
The new analysis estimated that between 0.4 percent and 1.4 percent of cases were fatal.
They said in an early outbreak in mid-February in the village of La Gloria, Veracruz, over half the population suffered acute respiratory illness, affecting more than 61 percent of under 15-year-olds in the community.
Using a variety of methods to estimate how easily the virus is transmitted, the researchers said that each case of the flu resulted in between 1.4 and 1.6 infections to others.
Data on the spread and strength of the illness is still incomplete, the researchers stressed. But they said their findings can help policymakers make such decisions as whether to close schools, balancing the cost of such actions against the potential to prevent spread of disease.
The potential spread of the illness in the Southern Hemisphere, which is just beginning its flu season, needs to be closely monitored, Ferguson's team wrote.
WHO's announcement of a Level 5 alert meant that a virus has caused sustained community level outbreaks in at least two countries in one region, and a worldwide pandemic is considered imminent. It alerts countries that do not have the illness yet to prepare for its arrival and institute their pandemic preparedness plans.
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